Most of us encountered to increase the limit of file descriptors while installing a high end 3pp(party product).In doing so we will encounter with two different kinds of limits i.e.soft vs hard.
Hard limits are a kernel-configurable item and users can't exceed them. Soft limits are the user defaults and users can change that using the ulimit command.
Basically, soft limits can be changed to anything up to the hard limit. Soft limits are warning barrier. When a user reaches the soft limit they will get an warning message but are still allowed to use more space up to the hard limit.
Since its a kernel tunable we have to define the value in /etc/system and /etc/sysctl.conf for Pre-Solaris 10 and Linux respectively.
E.g.,
To set a hard limit of 4096 and soft limit of 1024 in Solaris 8.
set rlim_fd_max=4096 [Refers Hard limit]
set rlim_fd_cur=1024 [Refers Soft limit]
To raise the allowed limit in Linux based distributions update either /etc/limits.conf or /etc/security/limits.conf
